In connected to a base station or acquiring a basic connection identifier (Basic CID), a mobile station perform handover to move into an area close to another base station or to be served as better quality of service. Handover can be initiated by a request of a base station or a mobile station. When a mobile station requests handover, handover is initiated through a mobile station's handover request (MOB_MSHO-REQ) message, and when a base station requests handover, handover is initiated through a base station's handover request (MOB_BSHO-REQ) message.
FIG. 1 illustrates a flow of messages in process of handover initiated by a mobile station.
After a mobile station decided to perform handover based of strength of signals of neighbor base stations (100), it transmits a handover request (MOB_MSHO-REQ) message to a serving base station (110). The serving base station is a base station connected to the mobile station.
The serving base station transmits a handover response (MOB_BSHO-RSP) message, which includes a recommended base stations list, to the mobile station (120).
The mobile station selects a base station for handover referring the recommended base stations list and a list that the mobile station has (130), and transmits a handover indicate (MOB_HO_IND) message to the serving base station (140).
FIG. 2 illustrates a flow of messages in process of handover initiated by a base station.
After a serving base station decides that a mobile station must perform handover, it transmits a handover request (MOB_BSHO-REQ) message, which includes a recommended base stations list, to a mobile station (200, 210).
The mobile station selects a base station for handover in a recommended base stations list, or decides to refuse a handover request of the serving base station. And the mobile station transmits a handover indicate (MOB_HO_IND) message (240). The handover indicate message includes information of the selected base station or the decision of refusing a handover request.
A serving base station makes a recommended base stations list based on information of neighbor base stations collected through a network and transmits it to a mobile station. Then the mobile station decides a target base station referring to the recommended base stations list. But the mobile station can perform handover to a base station which is not included in the recommended base stations list and can not serve a mobile station because of restart caused by an error or overload. If a mobile station performs handover to a base station that has overload or is expected to restart, the service of the base station will be terminated after handover.
FIG. 3 illustrates a case in that a service for a mobile station is terminated because of a base station's problem after handover.
Referring to FIG. 3, after a mobile station decided to perform handover, it transmits a handover request (MOB_MSHO-REQ) message to a serving base station (310).
The serving base station transmits handover response (MOB_BSHO-RSP) message, which includes a recommended base stations list to the mobile station (320). In FIG. 3, base stations B, C and D are included in the recommended base stations list.
And a base station A(T-BS A) encounters or expected to encounter an error. The mobile station selects a base station for handover referring the recommended base stations list, and transmits a handover indicate (MOB_HO_IND) message to the serving base station (330). In FIG. 3, the handover indicate message includes information of the selected base station (T-BS A).
Even if the mobile station can perform network entry to the base station A (T-BS A) and be serviced by the base station A (T-BS A) (340), the service of the base station A (T-BS A) will be terminated when the base station A (T-BS A) restarts because of an error (350).
Whenever a base station restarts, it increases a parameter of restart count by 1, and sends the parameter of restart count to a mobile station through a downlink channel descriptor (DCD) or a super frame header (SFH). Comparing a value of the parameter of restart count with a previous value of a parameter of restart count, the mobile station can know whether the base station restarted or not. In case that the value of the parameter of restart count increased, the mobile station performs network entry again and registers the base station.
Thus, the mobile station performs network entry again after the base station A completes restart (360).